X-Ray Computed Tomography (CT) was introduced in the late 1970s as a means for forming three dimensional images of human anatomy. Although its initial spatial resolution was inferior to that of film radiography, it brought a new level of contrast resolution that enabled radiologists to discern previously undetectable low contrast pathology. The apparatus configuration evolved through many generations and is often configured using a rotating X-ray source which is opposed by a detector array rotating in fixed relationship to the source. The x-ray source is often mounted on a C-arm system or conventional gantry. The detector arrays often consist of two dimensional arrays of discrete detectors in conventional CT or in the form of a large area cone beam flat panel detector in C-Arm CT.
Conventional CT is used for a wide range of diagnostic tasks and generally rather scatter free signal detection due to the smaller area of the detector arrays, although these areas are increasing in recent years. C-Arm CT is typically used for interventional procedures where it has been recently possible to obtain 3D Digital Subtraction Angiographic (DSA) data reconstructions by performing a CT angiogram following the introduction of contrast (e.g., iodine contrast) into the vascular system. CT angiography can also be implemented on conventional CT systems but due to the small detector area, the injected contrast bolus must be followed and the timing of the gantry or table advanced relative to the bolus traversal, which can pose timing problems that result in images being obtained during suboptimal opacification.
In the 1980's the concept of spiral CT was introduced. In this mode, rather than obtaining one slice at a time, the table is advanced through the rotating gantry and the x-rays passed through the patient in a helical fashion. Using data interpolation, reconstruction of a series of CT images of sequential planes can be quickly obtained.
Imaging enhancement procedures have long been used to generate better quality and more useful medical subject images, e.g., for use in X-ray CT applications. One such method was described in an article titled, The Use of a General Description of the Radiological Transmission Image, Optical Engineering 13(2):134; 1974., hereby incorporated by reference in it's entirety. The following Taylor expansion was presented relating the value of a radiological transmission image at two points in the multi-variable space defined by coordinates, x, y, and z, radiation energy E and time t:
                    ⁢          Equation      ⁢                          ⁢      1                  I      ⁡              (                              x            ′                    ,                      y            ′                    ,                      z            ′                    ,                      E            ′                    ,          t                )              =                  I        ⁡                  (                      x            ,            y            ,            z            ,            E            ,            t                    )                    =                                                  ⅆ              I                                      ⅆ              x                                ⁢          Δ          ⁢                                          ⁢          x                +                                            ⅆ              I                                      ⅆ              y                                ⁢          Δ          ⁢                                          ⁢          y                +                                            ⅆ              I                                      ⅆ              z                                ⁢          Δ          ⁢                                          ⁢          z                +                                            ⅆ              I                                      ⅆ              t                                ⁢          Δ          ⁢                                          ⁢          t                +                                            ⅆ              I                                      ⅆ              E                                ⁢          Δ          ⁢                                          ⁢                      E            ⁢                                                  ⁢            \+            ⁢                                                                                                                                             ⅆ                        2                                            ⁢                      I                                                                                      ⅆ                        E                                            ⁢                                              ⅆ                        z                                                                              ⁢                  Δ                  ⁢                                                                          ⁢                  E                  ⁢                                                                          ⁢                  Δ                  ⁢                                                                          ⁢                  z                                +                                                                                                    ⅆ                        2                                            ⁢                      I                                                                                      ⅆ                        E                                            ⁢                                              ⅆ                        t                                                                              ⁢                  Δ                  ⁢                                                                          ⁢                  E                  ⁢                                                                          ⁢                  Δ                  ⁢                                                                          ⁢                  t                                +                                                                                                    ⅆ                        3                                            ⁢                                                                                          ⁢                      I                                                                                      ⅆ                        E                                            ⁢                                              ⅆ                        t                                            ⁢                                              ⅆ                        z                                                                              ⁢                  Δ                  ⁢                                                                          ⁢                  E                  ⁢                                                                          ⁢                  Δ                  ⁢                                                                          ⁢                  t                  ⁢                                                                          ⁢                  Δ                  ⁢                                                                          ⁢                  z                                                                        where the two points in variable space are related by(x′,y′,z′,E′,t′)=(x+Δx, y+Δy, z+Δz, E+ΔE, t+Δt).
Information corresponding to the various terms in the expansion may be accessed using suitable image subtraction techniques. For example, time subtraction (i.e., subtraction of images acquired at differing times) may be used to access information regarding the fifth term (including the time derivative) in the expansion of Equation 1.